Subprocess variables not available in data assignment menu

In my process, there is the definition of a variable at process level: "process_var". There is a multi instance subprocess with the definition of another variable at this subprocess level: "subprocess_var". Inside the subprocess I have a task with a data-in variable definition: "dataIn_var". What I want to do is to map the subprocess_var to the dataIn_var, but in the data assignment menu it is impossible to select the variable on subprocess level.